#include <iostream>

using namespace std;

int n, m, x;

int countblack(int n, int m)
{
	if (n <= 0 || m <= 0) return 0;

	if (n % 2 == 0 || m % 2 == 0) 
		return (n*m)/2;
	else 
		return 1 + (n*m)/2;
}

int count(int nn, int mm, int xx)
{
	if (nn <= 0 || mm <= 0) return 0;

	if (xx == 1)
		return countblack(nn, mm) - countblack(nn-2, mm-2);
	else
		return count(nn-2, mm-2, xx-1);
}

int main()
{
	cin >> n >> m >> x;
	cout << count(n, m, x);
	return 0;
}
